Hi Faisal,

for CMFB it should be sufficient if you act on the CM voltage in your folded cascode 1st stage while measuring your CM output at the output of the 2nd stage. The following paper shows an example for CMFB in a folded-cascode amplifier:

"A high-speed fully differential CMOS Opamp", by A.Younis, presented at the Midwest Symposium on Circuits and Systems 2000. You can get it from the IEEE website.

There's also some examples in "Design of Analog Integrated Circuits and Systems" by Laker and Sansen, McGraw-Hill in chapter 6. You may find some info in any advanced CMOS design textbook targeting amplifier design.

Paul

Hi Faisal,

To my experience it is very difficult to design a single stable CMFB circuit covering both stages of the opamp because you will effectively have three inverters in the loop that form a ring oscillator and it is hard to make such a circuit stable. It is possible to make it stable but by the expence of the CMFB bandwidth. By using two CMFB circuits (one for each stage) you will avoid this problem. B. Razavi in "Design of Analog CMOS Integrated Circuits" on page 324 also confirms this: "It is important to note that fully-differential two-stage opamps require two CMFB networks, one for the output of each stage".

Eugene.